To play high you must:
1. Relax and conserve strength in the lower register.
2. Play with a lip touching embouchure. I can lip buzz over double high c. Buzzing builds chops and teaches you the right mouth position.
3. Use A Lip Set Point. If I set for a g on top of the staff I can relax and pivot to play low G and F#. I can also add air and compression to go over double high c.
4. Use A Pivot.
5. Learn how and when to add compression. NOT just tighten your belly some. Really squeeze all around your torso. NOT chest.
6. Use Timed Breathing. If you take a full breath you can NOT get but about a 30% compression compared to a half breath.
7. Use LESS mouthpiece pressure. It only shuts the aperture and keeps the notes from coming out.
8. Play SONGS in the upper register. Learn to tongue.
That’s a partial list.